In the case of blending fine particles and a binder and using the blend for usage such as adhesive, exterior paint, hardcoat and antireflection film, studies are being made to enhance scratch resistance, strength of the cured product, adhesion to another material contacted, and the like.
In blending fine particles and a binder, it is necessary not to cause unnecessary aggregation of an inorganic material of the fine particles. One of the methods employed in general is a method of dispersing fine particles in a solvent having affinity for a binder, mixing the dispersion with a binder, and forming a film. In order to obtain a stable performance, the matter of importance is that the fine particles are stably dispersed in the solvent. Specifically, control of hydrophilicity, hydrophobicity or steric hindrance on the fine particle surface is important and in the case of inorganic oxide fine particles, a surface treatment with alkoxysilane is known. For example, Non-Patent Document 1 describes a method of dispersing inorganic particles in an organic solvent by using a silane coupling agent.
After a coating composition containing fine particles and a binder can be stably prepared, the composition is coated on a support for the purpose of achieving a desired performance. If the state of the fine particles in the coating film cannot be controlled, the objective performance such as scratch resistance, strength of the cured product, and adhesion to another material contacted may fluctuate. In particular, the coating composition containing fine particles having a particle size of 1 μm or less has a large specific area and therefore, aggregation is liable to occur in the process of drying the organic solvent due to strong interaction between fine particles. As a result, aggregation or phase separation may be caused in the coating film and this may adversely affect the performance of the coating film.
On the other hand, in a display such as cathode ray tube display (CRT), plasma display panel (PDP), electroluminescent display (ELD) and liquid crystal display (LCD), an antireflection film is generally disposed on the outermost surface of the display so as to reduce the reflectance by utilizing the principle of optical interference and thereby prevent the reduction in contrast due to reflection of outside light or projection of an image.
Such an antireflection film can be produced by forming a low refractive index layer having a proper thickness on the outermost surface and depending on the case, appropriately forming a high refractive index layer, a medium refractive index layer, a hardcoat layer and the like between the low refractive index layer and the support. In order to realize a low reflectance, a material having a refractive index as low as possible is preferably used for the low refractive index layer. Furthermore, since the antireflection film is used on the outermost surface, this film is expected to function as a protective film of the display and required to allow for less attachment of dirt or dust and have high scratch resistance. In order to realize high scratch resistance of a thin film having a thickness of around 100 nm, strength of the film itself and firm adhesion to the underlying layer are necessary.
The means for reducing the refractive index of a material includes introduction of a fluorine atom and reduction of the density (introduction of voids), but either means tends to impair the film strength and adhesion and decrease the scratch resistance. Thus, it has been difficult to achieve both low refractive index and high scratch resistance.
Patent Documents 1 to 3 describe a technique of introducing a polysiloxane structure into a fluorine-containing polymer, thereby decreasing the coefficient of friction on the film surface and improving the scratch resistance. This means is effective to a certain extent for the improvement of scratch resistance, but in the case of a film substantially lacking in the film strength and interface adhesion, sufficiently high scratch resistance cannot be obtained only by this means.
Patent Document 4 describes a technique of adding a silane coupling agent to a low refractive index layer material utilizing a fluorine-containing polymer, thereby greatly improving the scratch resistance. However, the silane coupling agent having a low boiling point disadvantageously volatilizes and dissipates in the coating and drying steps and needs to be added in an excess amount by taking into account the volatilization loss. Thus, there is a problem that a stable performance is difficult to obtain.
The related arts of the present invention are as follows.
Non-Patent Document 1: Ganryo Bunsan Gijutsu, Hyomen Shori to Bunsanzai no Tukaikata Oyobi Bunsansei Hyoka (Pigment Dispersion Technology, Use of Surface Treatment and Dispersant, and Evaluation of Dispersibility), compiled by Technical Information Institute Co., Ltd. (1999)
Patent Document 1: JP-A-11-189621 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
Patent Document 2: JP-A-11-228631
Patent Document 3: JP-A-2000-313709
Patent Document 4: JP-A-2003-222704